This description relates to signaling for push to talk.
A Push-To-Talk (PTT) feature on mobile phones allows a mobile phone to be used as a digital two-way radio. By pressing and holding a PTT button, a user can talk while one or more other uses can listen. PTT connects mobile phone users with each other within seconds and bypasses the call setup time delay required for normal phone dialing. PTT provides a half-duplex communication mode controlled by a button used to switch between voice transmission mode and voice reception mode.
PTT may be implemented to operate over a realization of a EV-DO Rev A standard (also written as 1xEV-DO Rev A or 1x Evolution-Data Optimized Revision A) or another similarly capable standard. EV-DO Rev A is a part of a family of standards by 3GPP2.
The 1xEV-DO protocol is an EVolution of the 1xRTT standard for high-speed data-only (DO) services and has been standardized by the Telecommunication Industry Association (TIA) as TIA/ELA/IS-856, “CDMA2000 High Rate Packet Data Air Interface Specification”, 3GPP2 C.S0024-0, Version 4.0, Oct. 25, 2002, which is incorporated herein by reference. Revision A to this specification has been published as TIA/EIA/IS-856, “CDMA2000 High Rate Packet Data Air Interface Specification”, 3GPP2 C.S0024-A, Version 2.0, June 2005, which is also incorporated herein by reference.
Some implementations of PTT use an EV-DO Rev A messaging method called DataOverSignaling (DoS) to reduce the latency of push to beep, the time from when the caller pushes the PTT button to when the caller is granted the floor, (i.e., can begin speaking, also called “push to beep” latency). Data Over Signaling in EV-DO Rev-A is a mechanism to transmit “data packets” (i.e., packets whose contents are destined for higher-layer protocols) over a “control channel”. The control channel is an always-on link that is traditionally used to carry EV-DO signaling messages. When a user wishes to make a PTT call, the network sends a series of setup-messages between a handset (also referred to as an Access Terminal or AT) and various EV-DO network structures. These structures include Radio Access Networks (Access Network or AN), which are a portion of a mobile phone network that relates to the transmission between an AT and a network Base Station (BS), and Radio Network Controllers (RNC), which work with the BS to act as a link between the wireless devices (e.g. AT) and the Internet. DoS messages are passed between these devices to establish a link between the caller and the receivers.
When a user presses the PTT button on the AT to establish a connection, a message is sent from the AT to a PTT server located on a modem card of a BS. The PTT server then sends the message to an RNC to locate the target ATs. The message carries information designating the target AT. It then becomes the job of the RNC to locate the target ATs and properly establish the necessary link for a PTT connection.
The AN locates the AT by sending a small EV-DO specific signaling message, called the “page” message, over the always-on signaling control channel. The always-on signaling control channel connects the AN to sectors distributed over a geographic area where the AT is presumably located. The page message for a specific AT is sent at one or more specific times. These specific times are pre-agreed between each individual AT and the access network. At these pre-assigned times, the AT switches on its receive function and listens for messages addressed to itself. If the AT receives the page message, the AT then establishes an EV-DO connection with the RAN, over which it can receive user traffic.
Establishing such connections takes time. A long connection time detrimentally affects end-to-end “Push To Beep” time and negatively affects the user-experience.
In establishing the PTT link, some DoS messages are Session Initiation Protocol (SIP) invitations or acknowledgements (SIP INVITES/OK) for Mobile-Originated PTT (MO PTT) calls and Mobile-Terminated PTT (MT PTT) calls. In order to send the PTT message to the AT faster than would be possible with the traditional mechanism described earlier, the RAN has to send the PTT initiation message as a DOS message over the control channel, along with the page message. An AT that needs to send a PTT initiation message can also send it before a connection has been set up, by sending the message as a DOS message over a low rate access channel that exists in the network prior to traffic channel creation to speed up PTT call initiation. Call set-up messages (e.g. SIP INVITE) can be sent over the access channel, allowing for fast initiation of PTT calls. Without this mechanism, each PTT call would have to begin with the establishment of a traffic channel, and this would lead to long set-up times.
While a delay of several seconds in connecting to a server may not be noticeable to some applications, the delay may be noticeable for real-time applications such as voice and PTT. When not active, typical cellular handsets may reduce power usage by entering a sleep mode that may last as long as five seconds between awake states. This increases the latency for call set-up for PTT calls. EV-DO Rev A reduces the latency with the introduction of an Enhanced Idle State Protocol (EISP) that allows shorter sleep periods to speed up call establishment. Enhanced Idle State Protocol supports faster, variable paging.
In using DoS messaging for PTT, a typical vendor could bundle a forward link DoS message (encapsulating a SIP invite) along with a Page message. The forward link DoS message links a fixed location such as a radio node (RN) to a mobile user. A Page message is a message transmitting the ID of another device in order to establish connection with that device. Because the Page message is carried as a part of a synchronous sleep state (synSS) capsule sent on the control channel, the DoS message will also be transported in synSS. If the recipient AT is located in a sector, a network topology subdivision, the recipient AT will receive the DoS message along with the Page message. Typically, using the DoS message, a Page message is flooded to all sectors on a subnet.
For a typical IP_based push-to-talk system, even the smallest message would be 50-60 bytes in length. Commonly used SIP-based push-to-talk systems have messages that are hundreds of bytes long. Special SIP compression techniques are used to limit the initial “invite” messages to a size of less than 120 bytes. For a system that has 300 sectors in a subnet, and a modest PTT load, the control channel capacity (which is defined to be 6 DoS messages/sector/CC cycle at 76.8 Kbps) will constrain the maximum average number of PTT subscribers supported per sector, the average push to beep latency, and a limited maximum average simultaneous PTT calls per sector.